The relatively new technology of fiber optic cable has provided society with capabilities not previously envisioned. Fiber optic cable has numerous uses. In certain applications, it can be used for directing light along tortuous transmission paths. Consequently, it can be used in structures as diverse as surgeon's headlamps and devises for testing integrated circuit chip carriers wherein it is necessary to sense the presence of a device to be tested at a particular location. The fiber optic cable, because of its ability to transmit light in paths other than straight, can be used to ascertain whether a device is or is not in a particular location. If the device is sensed as being in a particular location because of a break in a light circuit transmitted along fiber optic cable and across a gap occupied by the device, certain functions can be performed.
Other more sophisticated applications exist for the use of fiber optic cable. A significant use to which it can be put is the transmission of telecommunications signals. The unique characteristics of fiber optic cable enable it to be used in such an application.
Similarily, it can be used to perform various telemetry-like functions. Such functions can include monitoring of voltage in power transformers and the transmission of information with regard to such voltages, etc.
With these more sophisticated applications, frequently significant lengths of such cable must be laid. Typically, such lengths of cable are threaded through a conducting conduit lubricated to facilitate transmission of the cable along the conduit length. In installing the cable in place in the conduit, a lead wire is used to pull the cable through the conduit.
Because the properties of the cable can be distorted if it is subjected to inordinate amounts of force during installation, it is important to know whether, while the cable is pulled through the conduit, it is subjected to forces in excess of acceptable limits.
To date, such tensions have been measured by use of an instrument secured to one end of a lead wire, while the other end of the lead wire was secured to the cable head by means of a swivel joint. The lead wire, in turn, is fed about a capstan, and the capstan transforms its rotational motion into linear movement of the cable along the axis of elongation of the conduit into which it is being pulled. By use of such an arrangement, the tension is actually being measured in a direction from the capstan different than the direction from which the cable is approaching. As a result of the looping of the lead wire about the capstan, the reading measured by a tensiometer does not accurately reflect the tension to which the cable was being subjected. Because much of the tensional force is absorbed as a result of the friction between the lead wire and the capstan outersurface, the reading generated by the tensiometer will be less then the actual tension applied to the cable.
The maximum acceptable tension will vary depending upon the size of the particular fiber optic cable being pulled through the conduit. For example, a standard size might function to effect its designed purpose if the maximum tension exerted upon it were no greater than, for example, 800 pounds. In order to provide a margin for error, it might be arbitrarily determined that any reading of the tensiometer indicating a tension in excess of 600 pounds would reflect a situation indicative of cable potentially damaged and not able to perform its function. With measurements performed in accordance with prior art operations, the tensiometer might well read a tension of 500 pounds when, in fact, the maximum tension to which the cable was subjected exceeded 800 pounds.
Larger cable may well be deemed to have received no damage if it does not have a tension greater than 1000 pounds exerted upon it. With such cable, similar circumstances can be envisioned wherein the actual recording by the tensiometer would be less than 1000 pounds while the actual tension to which the cable was subjected might have exceeded, for example, 1200 pounds.
Another problem existent in the prior art is one wherein, although the tensiometer used to measure the tension applied to the cable provides a reading as to tension at any particular time, there is no preservation of the maximum tension reading. In the particular application of pulling fiber optic cable through a conduit, the only truly significant reading is the maximum tension to which the cable has been subjected. It is, of course, difficult to constantly view the representation of the tension being sensed because the tensiometer is moving as the cable is drawn through the conduit. More importantly, however, it is not practicable to even try to continuously observe the tensiometer.
It is to these problems in the prior art, therefore, that the present invention is directed. The present invention comprises a method of accurately ascertaining the maximum tension to which the cable being drawn through a conduit is subjected and for preserving that reading for later consideration. It also includes an apparatus for accomplishing these goals.